Over the past twenty years, tetraethoxysilane (TEOS) has been extensively used as a liquid source for the deposition of silicon dioxide. Low pressure chemical vapor deposition (LPCVD) techniques have in general been employed in which TEOS is thermally decomposed at approximately 725.degree. C. in the presence of oxygen. More recently, plasma-enhanced chemical vapor deposition (PECVD) techniques, using either microwave or radio-frequency (rf) energy sources, have been employed to facilitate the decomposition of TEOS in the presence of oxygen at temperatures down to 20.degree. C.
For both LPCVD and PECVD techniques, the overall decomposition reaction can, in general, be represented by the following equation: ##STR1## If this oxidation process is incomplete, carbon monoxide may also be a product and the resulting oxide films may contain mobile ions such as carbon, hydroxyl and silicon-hydrogen groups.
As disclosed in co-pending Patent Application Ser. No. 07/133,757 filed 12/17/87, the compressive stress of silicon dioxide films using silane and nitrous oxide can be reduced by the addition of small amounts of fluorine-containing compounds such as CF.sub.4, SiF.sub.4, and NF.sub.3 In the case of SiF.sub.4, the deposition rate increased with increasing flow of the additive while with NF.sub.3 the rate decreased. This latter observation is not surprising since it is well established that NF.sub.3 is an excellent oxide etchant that produces .cndot.NF.sub.2 and .cndot.F radicals in an rf plasma environment.
For single-wafer plasma-enhanced chemical vapor deposition (PECVD) systems, it is essential to have a high deposition rate, usually above about 3000 Angstroms per minute, in order to compete with batch systems. This is conventionally obtained using pressures of two torr and higher. However, these relatively high pressures result in extensive gas phase nucleation and cause a "snow" of SiO.sub.2 throughout the reaction chamber. The tendency to create "snow" has conventionally been counteracted by elaborate inert gas "curtains" to confine the deposition, together with frequent in-situ cleaning of the chamber. The conventional low pressure reaction is very clean, but the deposition rate is unacceptable for single-wafer reactors. Therefore, a need has arisen for a clean, single wafer, PECVD system for the deposition of SiO.sub.2 with an acceptably high deposition rate.